1. Field of the Invention
The present invention relates to an objective lens element used for performing at least one of recording, reproducing, or erasing of information on an information recoding surface of an optical information storage medium, and an optical pickup device including the objective lens element.
2. Description of the Background Art
In recent years, researches and developments have been actively carried out concerning high-density optical discs that have an increased recording density by using a blue laser beam with a wavelength of about 400 nm and thus have an improved storage capacity. One of the standards of such high-density optical discs is Blu-Ray Disc (registered trademark; hereinafter, referred to as “BD”) in which the image side numerical aperture (NA) of an objective lens is set to about 0.85 and the thickness of a protective base plate on an information recoding surface of an optical disc is set to about 0.1 mm.
Other than BD, DVD (protective base plate thickness: about 0.6 mm) for which a red laser beam with a wavelength of about 680 nm is used, and CD (protective base plate thickness: about 1.2 mm) for which an infrared laser beam with a wavelength of about 780 nm is used also exist. Various objective lenses that are compatible with three types of standards of these discs have been proposed.
For example, Japanese Patent No. 3993870 discloses an optical element and an optical pickup device that are compatible with the three types of the standards of BD, DVD, and CD. An objective lens disclosed in Japanese Patent No. 3993870 is provided with a stair-like diffraction structure (also referred to as binary type diffraction structure) in which stair-like steps are periodically arranged. The height of each step is set such that a difference in optical path of about 1.25 wavelengths is provided to a light beam having a shortest designed wavelength. In addition, one periodic structure consists of four steps that are consecutive in a radial direction (the height from a base surface is 0 to 3 times that of a unit step).
Since such a step structure is provided, the diffraction efficiency of a +1st order diffracted light beam can be at its maximum when a light beam of a wavelength for BD is used, and the diffraction efficiency of a −1st order diffracted light beam can be at its maximum when a light beam of a wavelength for DVD is used. Thus, use of change in an angle of diffraction with respect to a wavelength makes it possible to compensate for a spherical aberration that occurs due to differences in wavelength and disc base material thickness when changing between BD and DVD.
Further, Japanese Laid-Open Patent Publication No. 2005-243151 discloses an objective lens that is compatible with DVD and a high-density optical disc for which a violet light semiconductor laser is used. The objective lens disclosed in Japanese Laid-Open Patent Publication No. 2005-243151 is provided with a plurality of ring-shaped steps on an optical function surface thereof. The depth of each ring-shaped step is set such that the diffraction efficiency of a +3rd order diffracted light beam is at its maximum when a light beam of a wavelength for the high-density optical disc is used and the diffraction efficiency of a +2nd order diffracted light beam is at its maximum when a light beam of a wavelength for DVD is used.
When the numerical aperture (NA) for the high-density optical disc is higher than the NA for DVD, the outer circumferential portion of the optical function surface of the objective lens is a region dedicated for the high-density optical disc. Thus, it is necessary to prevent a light beam for DVD that is incident on the region from contributing to spot formation. When the light beam for DVD that is incident on the region dedicated for the high-density optical disc is converged on an information recoding surface of an optical disc, it leads to deterioration of spot performance. However, in Japanese Laid-Open Patent Publication No. 2005-243151, designing that takes into consideration the difference in NA between the optical disc standards, such as limiting convergence of the light beam for DVD that is incident on the region dedicated for the high-density optical disc, is not performed. Therefore, in order to adjust an effective NA, an additional optical component having an aperture limiting function is required, but it is not preferred to provide such a component, since the number of parts and the cost are increased.